Method of partially plating substrate for electronic devices

ABSTRACT

The object of the present invention is to provide a free and precise control of the plating amount while easily determining a selected portion to be plated.  
     Small balls  24  are arranged at, and adhered or bonded to, via holes  22  of a TAB tape  21  and the small balls  24  are then melted so that a copper wiring  23  exposed at the via holes  22  of the TAB tape  21  can be selectively plated with a different metal to enable selected portions of a substrate for electronic devices to be partially plated easily and precisely.

TECHNICAL FIELD

[0001] The present invention relates to a method of partially plating asubstrate for electronic devices, and is particularly suited to a methodof partially plating selected portions, selectively, on a substrate forelectronic devices on which semiconductor products are to be mounted.

BACKGROUND ART

[0002] In order to protect metallic portions of a substrate forelectronic devices and easily bond the substrate to other electronicdevices, selected portions of the metallic portions have heretofore beenpartially plated. There are various types of plating methods, andtypical plating methods are wet electroplating and electroless plating.

[0003] Wet electroplating is conducted by covering or masking portionsother than selected portions in advance, and partially plating theselected portions. Wet electroplating, therefore, has the followingproblems: application to fine portions on a substrate for electronicdevices on which semiconductor products are to be mounted is notsuitable; and control of the plating amount is difficult.

[0004] Moreover, wet plating has another problem in that plating cannotbe performed unless the article to be plated is made electricallyconductive. Furthermore, since wet electroplating uses large amounts ofsolutions such as a plating solution and a cleaning solution, itrequires a large scale treating facility, which tends to cause anenvironmental problem.

[0005] On the other hand, since the electroless plating is conductedwith chemicals such as an acid or an alkali without utilizingelectrolytic reactions, there arises the problem that the metals whichcan be used for plating are restricted.

[0006] Furthermore, in order to plate selected portions alone, theselected portions must be subjected to surface treatment; therefore,electroless plating has the problem that it requires many elaborateprocedures. Still furthermore, electroless plating has disadvantages inthat it is difficult to form a thick plated layer and that the platingamount varies.

DISCLOSURE OF INVENTION

[0007] In view of the problems as mentioned above, the object of thepresent invention is to provide free control of the plating amount whileeasily determining a selected portion to be plated.

[0008] A method of partially plating a substrate for electronic devicesaccording to the present invention comprises arranging small balls atselected portions of a substrate for mounting semiconductor devicesthereon, or a substrate for mounting electronic devices thereon andincluding a lead frame, and adhering or bonding the small balls thereto,and melting the small balls, thereby selectively plating the selectedportions of the substrate for electronic devices with a different metal.

[0009] Moreover, the method of partially plating a substrate forelectronic devices according to the present invention is characterizedby that the method comprises provisionally arranging and holding thesmall balls on an arrangement base plate having through holes providedat positions corresponding to the portions to be plated of the substratefor electronic devices, transferring the arrangement base plate abovethe substrate for mounting electronic devices, and adhering or bondingthe small balls provisionally arranged at and held by the through holesto the portions to be plated, respectively.

[0010] Furthermore, in the provisionally arranging and holdingprocedure, excess small balls adhering to the arrangement base plate orthe small balls which are provisionally held by the substrate areremoved by applying vibrations to the arrangement base plate, therebyprovisionally arranging and holding the small balls.

[0011] Moreover, the vibrations are ultrasonic vibrations.

[0012] The present invention is further characterized in that the smallballs are selected from solder, Sn alloy or In alloy, and that theselected small balls are melted by reflowing to selectively plate theselected portions of the substrate for electronic devices with adifferent metal.

[0013] Moreover, the present invention is characterized in that thesmall balls are selected from Au, Ag, Pd, Pt, Ni or Cr, and that theballs are melted by partial heating.

[0014] The present invention is further characterized in that thesubstrate for electronic devices is an insulating resin substrate or apolyimide tape, and that the selected portions are wiring composed ofcopper.

[0015] Furthermore, the present invention is characterized in that thesubstrate for electronic devices is made of a ceramic material, and thatthe selected portions are wiring composed of copper.

[0016] Still furthermore, the present invention is characterized in thatthe substrate for electronic devices is a lead frame composed of copperor iron alloy, and that the leads of the lead frame are partiallyplated.

[0017] Since the present invention comprises the technological means asmentioned above, metal balls provisionally arranged at selected portionsof a substrate for electronic devices are melted, and as a result themetal balls are thermally diffused to produce firm adhesive force withthe substrate metal. Since the metal balls are bonded to the substratemetal by thermal diffusion, it is desirable that the metal balls beexcellent in bonding caused by diffusion.

[0018] In addition, when a combination of the ball metal and thesubstrate metal shows a poor bonding force, or when the diffusion rateof the ball metal is too large, it is preferred to allow a metaldifferent from the substrate metal and the ball metal to intervenebetween both metals.

BRIEF DESCRIPTION OF DRAWINGS

[0019]FIG. 1 shows a first embodiment of the present invention, and is aplan view of a glass-epoxy substrate which has leads of copper wiringformed thereon.

[0020] FIGS. 2(a), 2(b) are views for illustrating the arrangement ofsmall balls.

[0021]FIG. 3 shows a second embodiment of the present invention, and isa view illustrating a method of partially plating via holes of a TABtape with solder.

[0022]FIG. 4 shows a third embodiment of the present invention, and is aview illustrating an example of a lead frame.

BEST MODE FOR CARRYING OUT THE INVENTION

[0023] One embodiment of the method of partially plating a substrate forelectronic devices of the present invention will be explained below withreference to drawings.

[0024]FIG. 1 shows a first embodiment of the present invention, and is aview for illustrating partial plating of selected portions (ends) 3 ofleads of copper wiring 2 on a glass-epoxy substrate 1.

[0025] The leads of copper wiring 2 in FIG. 1 have a wiring width of 50μm. Balls 4 used for partially plating the selected portions 3 aresolder balls, and have a diameter of 60 μm (eutectic solder).

[0026] When the method of partially plating the substrate for electronicdevices in the present embodiment is carried out, firstly, the balls 4are provisionally arranged at the ends 3 of the leads of copper wiring 2on the glass-epoxy substrate 1.

[0027] Next, the balls 4 are reflowed at 290° C. so that the selectedportions 3 of the leads of copper wiring 2 are plated with the balls 4to achieve solder plating.

[0028] Next, a procedure of provisionally arranging the balls 4 at theends 3 of the leads of copper wiring 2 will be explained with referenceto FIGS. 2(a), 2(b).

[0029] As shown in FIGS. 2(a), 2(b), the balls 4 are arranged in thefollowing manner. The back side of a 0.3 mm thick arrangement base plate13 having holes 11, which have a diameter of 40 μm, pierced through theplate at positions corresponding to the selected portions 3 of the leadsof copper wiring 2 formed on the glass-epoxy substrate 1 is attracted bysuction by applying a vacuum pressure (attracting mechanism not shown).The arrangement base plate 13 is brought close to a container 10accommodating the balls 4 while the attracted state is being maintained.

[0030] The balls are attracted by suction and held at the respectiveholes 11. Excess balls adhere to portions other than the holes 11 of thearrangement base plate 13, or other excess balls adhere to the balls 4attracted by suction to the holes 11 during attracting the balls bysuction; therefore, the excess balls must be removed. In order to removethe excess balls, an arbitrary procedure such as a procedure in whichvibrations are applied can be utilized. For example, the excess ballscan be preferably removed by applying ultrasonic vibrations to thearrangement base plate 13 in the horizontal direction.

[0031] The selected portions 3 of the leads of copper wiring 2 and theholes 11 of the arrangement base plate 13 are subsequently relativelymoved so that the holes 11 are located in correspondence with theselected portions 3, respectively, and the balls 4 are provisionallyadhered to the selected portions 3 of the leads of copper wiring 2,respectively. The balls 4 are then melted by reflowing and are bonded tothe selected portions 3 of the leads of copper wiring 2 as explainedabove.

[0032] Next, a second embodiment of the method of partially plating asubstrate for electronic devices of the present invention will beexplained with reference to FIG. 3.

[0033] In the second embodiment, via holes 22 of a TAB tape 21 areplated with solder.

[0034] As shown in FIG. 3, small balls 24 are provisionally arranged oncopper (Cu) wiring 23 exposed at a plurality of the via holes 22 formedat given positions of the TAB tape 21. In this case, the via holes 22have a diameter of 100 μm, and the small balls 24 have a diameter of 60μm. The number of the via holes 22 is, for example, about 300, and theyare formed as one unit in a lattice-like form.

[0035] When a plurality of the small balls 24 are provisionally arrangedas explained above, they are subsequently reflowed, whereby the copper(Cu) wiring 23 exposed at the via holes 22 can be plated with soldereasily and precisely.

[0036] Therefore, according to the method of partial plating in thepresent embodiment, wiring having a melting point lower than that of theTAB tape 21 can be formed at the via holes 22. As a result, the copper(Cu) wiring 23 exposed in the via holes 22 of the TAB tape 21 and theelectrodes of a semiconductor chip (not shown in the figure) can bebonded together at low temperature. It becomes, therefore, possible toarrange electrodes in an area array-like form on an insulating tape.

[0037] Accordingly, the method of partially plating a substrate forelectronic devices of the present embodiment is very favorable to thebonding of multi-pins. Moreover, since the TAB tape 21 and the smallballs 24 can be bonded together collectively using the arrangement baseplate 13, the method can greatly improve the productivity of a largerange of high density devices having a number of electrodes.

[0038] Next, a third embodiment of the present invention will beexplained with reference to FIG. 4.

[0039] In the third embodiment, the method of partial plating is appliedto plating only the leads of a lead frame 31 with silver.

[0040] In the present embodiment, a silver ball (not shown in thefigure) having a diameter of 150 μm is arranged at the end of each ofthe lead electrodes having a lead width of 150 μm, and provisionallyadhered to the end. The silver ball is then spot-irradiated with a laserbeam to be melted so that the end of the lead electrode is platedtherewith.

[0041] Since partial plating is conducted as explained above in themethod of partial plating in the present embodiments, selected portionsof a substrate for electronic devices can be selectively plated with adifferent metal efficiently, and the plating amount can be controlledprecisely.

[0042] That is, balls are arranged at selected portions of a substratefor electronic devices in the following manner. The back side of anarrangement base plate having holes formed at positions corresponding tothe selected portions is attracted by suction by applying a vacuumpressure, whereby the balls are attracted by suction to and held by theholes, respectively. For example, about 300 balls can be arrangedcollectively.

[0043] Moreover, the plating amount of a selected portion can becontrolled easily and precisely by adjusting the size of the ball.Furthermore, any metal can be used for plating so long as preparation ofsmall balls of the metal is possible. The plating method also has anadvantage of causing no environmental problem at the time of plating.Still furthermore, the partial plating method of the present embodimentscan be easily applied to plating two or more different layers inlamination.

[0044] Industrial Applicability

[0045] As explained above, small balls are provisionally arranged atportions to be plated, and melted so that the portions are plated withthe metal in the present invention. Accordingly, the present inventionmarkedly improves the plating efficiency compared with conventionalprocedures where plating is conducted by depositing metal on an atomicor molecular scale.

[0046] Moreover, since the selectivity of portions to be plated isexcellent, desired positions can be freely plated without conductingprocedures such as masking portions not to be plated, and the platingoperation can be greatly rationalized.

[0047] Moreover, the present invention has excellent advantages such asexplained below. The plating amount can be easily and preciselycontrolled by adjusting the size of the balls. Furthermore, there is nofear of fouling the substrate for electronic devices and plating can beconducted without environmental pollution because no plating solutionsare used.

1. A method of partially plating a substrate for electronic devices,comprising arranging small balls at selected portions of a substrate formounting semiconductor devices thereon, or a substrate for mountingelectronic devices thereon and including a lead frame, and adhering orbonding the small balls thereto, and melting the small balls, therebyselectively plating the selected portions of the substrate forelectronic devices with a different metal.
 2. The method of partiallyplating a substrate for electronic devices as claimed in claim 1 ,wherein the method comprises provisionally arranging and holding thesmall balls on an arrangement base plate having through holes providedat positions corresponding to the portions to be plated of the substratefor electronic devices, transferring the arrangement base plate abovethe substrate for mounting electronic devices, and adhering or bondingthe small balls provisionally arranged at and held by the through holesto the portions to be plated, respectively.
 3. The method of partiallyplating a substrate for electronic devices as claimed in claim 2 ,wherein, in the provisionally arranging and holding procedure, excesssmall balls adhering to the arrangement base plate or the small ballswhich are provisionally held by the substrate are removed by applyingvibrations to the arrangement base plate, thereby provisionallyarranging and holding the small balls.
 4. The method of partiallyplating a substrate for electronic devices as claimed in claim 3 ,wherein the vibrations are ultrasonic vibrations.
 5. The method ofpartially plating a substrate for electronic devices as claimed in claim1 or 2 , wherein the small balls are selected from solder, Sn alloy orIn alloy and the selected small balls are melted by reflowing toselectively plate the selected portions of the substrate for electronicdevices with a different metal.
 6. The method of partially plating asubstrate for electronic devices as claimed in claim 1 or 2 , whereinthe small balls are selected from Au, Ag, Pd, Pt, Ni or Cr, and theballs are melted by partial heating.
 7. The method of partially platinga substrate for electronic devices as claimed in claim 1 or 2 , whereinthe substrate for electronic devices is an insulating resin substrate ora polyimide tape, and the selected portions are wiring composed ofcopper.
 8. The method of partially plating a substrate for electronicdevices as claimed in claim 1 or 2 , wherein the substrate forelectronic devices is made of a ceramic material, and the selectedportions are wiring composed of copper.
 9. The method of partiallyplating a substrate for electronic devices as claimed in claim 1 or 2 ,wherein the substrate for electronic devices is a lead frame composed ofcopper or iron alloy, and the leads of the lead frame are partiallyplated.